1. Field
The present invention relates generally to data communication, and more specifically to adaptive rate control techniques for a wireless (e.g., OFDM) communication system.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), or some other multiple access technique. Some systems may also implement orthogonal frequency division modulation (OFDM), which may be capable of providing high performance for some channel environments.
In an OFDM system, the system bandwidth is effectively partitioned into a number of (NF) sub-bands (which may be referred to as frequency bins or subchannels). Each frequency subchannel is associated with a respective subcarrier upon which data may be modulated, and thus may be viewed as an independent “transmission channel”. Typically, the data to be transmitted (i.e., the information bits) is encoded with a particular coding scheme to generate coded bits, and the coded bits may further be grouped into non-binary symbols that are then mapped to modulation symbols based on a particular modulation scheme (e.g., QPSK, QAM, or some other scheme). At each time interval that may be dependent on the bandwidth of each frequency subchannel, a modulation symbol may be transmitted on each of the NL frequency subchannels.
The frequency subchannels of an OFDM system may experience different channel conditions (e.g., different fading and multipath effects) and may achieve different signal-to-noise-plus-interference ratios (SNRs). Consequently, the number of information bits per modulation symbol (i.e., the data rate) that may be transmitted on each frequency subchannel for a particular level of performance may be different from subchannel to subchannel. Moreover, the channel conditions typically vary with time. As a result, the supported data rates for the frequency subchannels also vary with time.
A key challenge in a coded OFDM system is the selection of an appropriate “rate” to be used for a data transmission based on channel conditions. This typically involves the selection of a particular data rate, coding scheme (or code rate), and modulation scheme. The goal of the rate selection should be to maximize throughput while meeting quality objectives, which may be quantified by a particular frame error rate (FER), certain latency criteria, and so on.
One straightforward technique for selecting the rate is to “bit load” each frequency subchannel according to its transmission capability, which may be quantified by the subchannel's short-term average SNR. However, this technique has several major drawbacks. First, encoding individually for each frequency subchannel can significantly increase the complexity of the processing at both the transmitter and receiver. Second, encoding individually for each frequency subchannel may greatly increase coding and decoding delay. And third, a high feedback rate may be needed to send channel state information (CSI) indicative of the channel conditions for each frequency subchannel (e.g., the gain, phase, and SNR, or the rate for each frequency subchannel). The feedback from the receiver would be needed to allow the transmitter to properly code and modulate the data on a subchannel-by-subchannel basis.
The different transmission capabilities of the frequency subchannels plus the time-variant nature of the communication channel make it challenging to effectively code and modulate data for transmission in an OFDM system. There is therefore a need in the art for techniques to adaptively control the rate of a data transmission in an OFDM communication system.